Heat recovery ventilators or HRVs are known. In recent years, building construction practice standards have become increasingly more rigid in light of a greater public awareness of environmental concerns and a desire for improved energy efficiency. Particularly in colder climates, energy efficient buildings are in demand in light of escalating energy costs and a desire for responsible use of energy resources. In Canada, this demand has led to the development a benchmark building practice and standard known as R2000. R2000 defines a set of guidelines and practices that a builder follows to ensure that the resulting structure is well insulated, energy efficient and, substantially air tight, i.e.--designed to minimize air infiltration/exfiltration through windows, doors and cracks.
One concern with energy efficient buildings such as the R2000 structure is that since the buildings are designed to substantially eliminate air infiltration/exfiltration, factors such as cooking and body odours, germs and depletion of oxygen through normal breathing result is poor indoor air quality. Occupants subjected to poor quality air conditions often complain of frequent illness or excessive fatigue. Further, modem day construction practices employ a wide range of materials and chemicals which tend to contradict these environmentally responsible/energy efficient intentions. A multitude of modem day materials are employed regularly that have been found to be potentially toxic if allowed to accumulate in the air within a building, for example: dry wall joint compound used in finishing walls; synthetic fibres in new carpeting can slowly emit gases into a occupied space; construction adhesives; and paint chemicals are capable of off gassing years after they are installed. Also, in many locations a risk exists that Radon gas will seep into the basement or other substructures of a building.
In order to ensure adequate removal of the above-mentioned conditions/substances which detract from air quality, HRVs were developed to simultaneously exhaust stale air from building spaces and replace it with fresh air at a controlled rate without a substantial loss of heat energy. Conventional HRVs typically comprise: a housing; at least one air to air heat exchanger core; a blower; a stale air passage; a fresh air passage; and appropriate stale and fresh air inlets and outlets. When the building is being heated, by a furnace for example, the heat exchanger core is used to transfer waste heat from the exhausted warm stale air to the incoming cooler fresh air, without the fresh and stale airflows mixing. When the building is being cooled, by an air conditioner for example, heat energy is transferred from the incoming warmer fresh air to the exhausted cooler stale air. In this regard, HRVs provide a means to continually supply a building with fresh air in a relatively energy efficient manner.
When an HRV is used during the heating of a building in a cold climate, the stale air processed by the HRV usually contains a certain amount of moisture. One common problem encountered in these conditions is that the moist stale air can condense and/or freeze as heat is transferred from it within the HRV. This undesirable situation results in a decrease in the heat transfer efficiency of the HRV and, in extreme cases, can result in blockage of the exhaust path and/or damage to the HRV.
Previous attempts have been made to overcome the above-identified problem of condensation and/or freezing (hereinafter referred to as `frosting`). One prior art solution was to install a thermostatically controlled electric reheat coil in exhaust air path. Several disadvantages result when employing this type of solution. In particular, electrical heating coils are costly to operate and decrease the overall energy efficiency of the HRV.
Another method of defrosting an HRV is to circulate the warm stale air through the frosted passage in the heat exchange core prior to exhausting it. This method is typically accomplished with dampers which block the supply of incoming fresh air and cause the warm stale air to pass through both passages of the heat exchanger to defrost the HRV. In this mode of operation, no fresh air is available to replace the exhausted stale air, leading to the creation of a negative pressure in the building relative to the exterior atmospheric pressure. This negative pressure may cause undesirable infiltration through doors, windows and cracks. An even greater concern is that such a negative pressure can create a backdraft in the flue ducting of gas or oil fired heating equipment which can lead to combustion gases entering the building. Further to this end, the presently proposed 1995 amendments to the Canadian National Building Code, developed by National Research Council of Canada, specifically prohibit whole house depressurization.
U.S. Pat. No. 5,193,610 to Morissette et at. teaches a prior art method of defrosting an HRV which avoids formation of the above-described negative pressure situation. In this reference, two dampers are employed with the first damper being used to redirect warm stale air to both the fresh air inlet and the exhaust air inlet of the heat exchanger. The second damper simultaneously blocks the stale air outlet and redirects the stale air leaving the heat exchanger to the fresh air outlet to prevent any stale air exiting the building through the HRV. In this mode of operation, the HRV recirculates stale air to defrost the heat exchanger.
One problem with this type of arrangement is that the damper assemblies are costly to manufacture as each requires a motor, a damper gate, air seals and some type of control means. In some instances, the damper assemblies only include a single motor and a mechanical linkage is employed to operate multiple damper gates, but this is still relatively expensive to manufacture. Also, in either situation, each damper constitutes a moving part and an increase in the number of moving parts increases the likelihood that the HRV will suffer a mechanical failure at some point in its lifetime.
Another problem associated with conventional HRVs is that the blower assembly is usually difficult to access for required maintenance purposes and thus, preventative maintenance of the HRV may not easily be performed by the building occupant. This can lead to failure of the HRV and/or an accumulation of dust, etc. within it.